This invention relates to Energy Pulse Bonding -EPB- and more particularly to a bonding head for applying heat and pressure to electrical conductors which are to be bonded together.
In the fabrication of many electric devices, such as semiconductor devices, printed circuit board panels, flat or tape-type electrical cables and the like, it is often necessary to bond one or more electrically conductive leads to an electrically conductive terminal, lug, or other electrically conductive lead. Typically, such bonds are formed by hand soldering techniques or by other multi-step techniques which necessitate accurately positioning the element to be bonded, feeding a measured quantity of a conductive bonding material, liquifying the material, and properly distributing the material over the bond or connection site. However, regardless of the particular technique employed, it must be such as to assure the formation of a good electrical connection and a mechanically strong and durable bond.
Generally speaking, bonding techniques involving the use of solder often require elaborate means for properly controlling the quantity of solder being fed to a prospective bonding site and the distribution thereof. Additionally, solder bonding techiques often require a particular orientation of the elements being bonded with respect to the bonding means employed, are rather inflexible as to the number of elements that can be bonded simultaneously and, are often limited in their application to bonding elements having a particular size and shape. It will be appreciated that the inability of conventional solder bonding techniques to adequately control the feeding and distribution of solder has rendered them uneconomical, inefficient, and has often resulted in the formation of generally unreliable electrical connections. In an attempt to avoid the problems accompanying the feeding, liquification and distribution of various bonding materials, techniques employing precoated elements have been developed. Generally speaking, these techniques employ elements having a low melting or alloy (i.e., solder) or some other heat-softenable electrically conductive material coated thereon so as to avoid the need for feeding a bonding material to a prospective bond site from a remote source. Although these techniques have reduced a need for independently feeding a measured quantity of bonding materials; have reduced the quantity of material required to effect a bond; and have minimized the difficulties inherent with the elements to be bonded, they are generally incapable of forming mechanically strong bonds. Methods that have been developed to bond together two electrically conductive leads are fusion bonding or energy pulse bonding which contemplate the application of heat and pressure to two electrically conductive materials to bond them together. One example of such a process may be found in U.S. Pat. No. 3,591,755 entitled "Fusion Bonding" issued July 6, 1971 to R. H. Cushman. Fusion bonding is now used extensively for connecting together the electrical leads of thin film and integrated circuits which are commonly manufactured on relatively fragile substrate such as glass, ceramic, silicon or germanium. Also, tape type electrical cables, which are comprised of a sheet of plastic with thin (about 0.001 inches) ribbons of electrically conductive material imbedded therein, may also be connected together or to a circuit with such a fusion bonding process.
The American Welding Society defines "diffusion welding" as a solid state welding process wherein coalescence of the faying surfaces is produced by the application of pressure at elevated temperatures. The process does not involve macroscopic deformation or relative motion of the parts. This definition has been categorized as "diffusion controlled welding." A second category of diffusion welding which requires macroscopic deformation has been referred to as "deformation diffusion welding." Energy pulse bonding falls into this second category where macroscopic deformation at high temperatures is required to decrease bonding times from minutes or hours to seconds and to aid in dispersing surface insulation, contaminants and oxides.
Energy pulse bonding utilizes mechanical pressure and high temperature in the form of a controlled energy pulse of temperature and pressure applied to the junction of two electrical conductors for two to three seconds. The magnitude of the temperature and pressure is determined by the yield strength and melting point of the material with the major role of pressure being to flatten asperites and bring more area into contact, thereby resulting in a greater bulk diffusion across the bond interface. The pressure applied to the junction must be great enough to cause microscopic plastic deformation of the surface irregularities in order to minimize voids at the bond surface. If the bonding surfaces are very irregular and the pressure insufficient, there will be voids left at the interface that cause the formation of a weak joint. The temperature applied to the junction controls the migration of atoms across the bond interface. Therefore, with high bonding temperatures, good bonds can be obtained in a time interval of 2 to 3 seconds. A preferable bonding temperature used in an energy pulse bonding process is between 50 to 75 percent of the melting point of the materials to be bonded. How long the pressure and temperature should be supplied to the junction is of course dependent on the bonding temperature, pressure, and bonding head characteristics i.e., thermal conductivity. Higher pressures (above the yield strength of the material) favor short bonding times while bonding pressures below the yield strength of the material require longer bonding times, possibily several hours to obtain a reliable bond.
Since one parameter of energy pulse bonding is temperature (heat transfer), it is important that the bonding head have a high thermal conductivity. However, materials that exhibit high thermal conductivity generally have poor mechanical strength. Therefore, bonding heads that are comprised of a material having a high thermal conductivity maximize the transfer of heat from the heat source to the bonding surface but have a relatively short life due to mechanical failure, distortion or deformation because of the poor mechanical (tensile) strength of the material; and bonding heads that are comprised of a material having a high tensile strength do not have the thermal conductivity necessary to allow the bonding to be used effectively for energy pulse bonding.